Electrical or electronic systems may often contain a series of elemental devices connected to a master device, which receives electrical signals from the series of elemental devices. The master device may also transmit signals to the elemental devices. If one of the elemental devices becomes defective, the signals to or from the maser device may be interrupted or altered.
An interruption may cause the operation of the entire system to halt until the fault is corrected. Thus, the fault in the single device may cause the entire system to become idle. Time must be expended to identify the faulty device and to effect repairs before the system becomes operational again.
An unexpected alteration of the signal may cause the system to operate incorrectly. Many systems require the continued accurate operation of the system and, consequently, the rapid identification and repair of any defect which will affect the output of the system is of extreme importance. A fault in an elemental device may remain undetected for some time. Even though it may be known that a defect exists, the identification of the precise location of the defect may require a significant amount of time. Until such time as the fault is repaired, the system will either be inoperative or function improperly.
In order to avoid interruption and to protect the integrity of the signal, it is desirable to isolate and by-pass the defective device, until repairs are effected.
Common means for isolating and by-passing devices may make use of breakers or fuse-activated relay switches. When a fuse-activated relay switch is used, before the device and the system can be returned to normal operation, the fuse must be manually replaced. In some applications involving low currents, for instance in a system where sales and inventory information is fed from cash registers in a store to a computer, breakers and fuses lack sufficient sensitivity and are impractical to use. In addition, for such application fuses may be fragile.
Other means for isolating devices may examine the information carried by the signal from each device. When a fault occurs this information may indicate the approximate location of such fault. Isolation action using relay switches is based upon the said information. Isolator units of this sort are constructed to accomodate a fixed number of devices. Because the number of devices protected may be less than the number of devices for which the isolator unit was designed, these units may have a large, unused, unnecessary isolation capacity. Thus, while solving the problems associated with the use of breakers and fuses, these units can be very expensive. Furthermore, isolator units of this type require a single, fixed location, usually very close to the master device, such as a computer, which receives signals from and transmits signals to the elemental devices. Cables must be run from each elemental device to the isolator unit. Therefore, because of the single, fixed location of this isolator unit, cable costs can be significant.
Also, if the locations of the elemental devices are shifted, the cables must also be shifted. During such a move, cables are susceptible to damage. If a cable is damaged, part or all of the cable from the elemental device to the isolator unit may have to be replaced, thus further increasing cable costs.
With a view to overcoming these problems, the invention provides a method for isolating and by-passing a device, line or circuit, and an isolator station, or loop fault isolator, for implementing same. The isolator station comprises one or more isolator units and an interface unit. The station simultaneously monitors the acceptability, or quality, of both currents and voltages at points immediately before and immediately following each protected device. The measurements of current and voltage are performed in such fashion that there is no significant change in the measured signal. When the acceptability of the signal changes between the defined measuring points, a fault is present in the protected system. Depending on the specific changes in the signal, the approximate location of the fault can be identified and the faulty device isolated and by-passed. The acceptability of the signal changes if the current, the voltage, or both, do not fluctuate within and up to the limits of a predetermined, acceptable range. These faults correspond to open circuits, short circuits, or other faults in the protected system.
When the fault conditions persist beyond a predetermined delay period, the isolator station will operate to isolate and by-pass the defective device by activating relay switches. The delay period allows the system to withstand transient conditions, such as those occurring when a device is switched on or off, without triggering the isolator unit.